1. Field of the Invention
This invention concerns discharge lamp ignition devices for starting discharge lamps, particularly high-intensity discharge lamps, such as mercury lamps, metal halide lamps, and xenon lamps.
2. Description of Related Art
High intensity discharge lamps HID lamps are used as light sources for optical equipment used for displaying graphic images, such as liquid crystal projectors and DLP® projectors. One method used in these projectors for displaying color images is to split the three colors-red R, green G, and blue B-using a dichromic prism or other means, to generate three separate images with a space modulation element for each color, and then, to recombine the light paths using a dichromic prism or other means. Another method for displaying color images is to spin a filter that comprises a color wheel that passes the three primary colors R, G, B to sequentially generate three colored luminous fluxes by passing light from the light source through this filter, which is a dynamic color filter, and then, to sequentially generate images in the three colors by time division by means of controlling the space modulation element in synchronization with the filter.
Among the discharge lamp ignition devices that start the discharge lamps described above, there are those which, with the voltage called the no-load discharge voltage impressed on the lamp at startup, impress a high voltage to generate dielectric breakdown within the discharge space to bring about first a glow discharge, then an arc discharge, and finally the stable steady voltage. The glow discharge generally has a higher voltage than the arc discharge, and is a transitional discharge that continues until the electrode temperature is sufficient to bring about the arc discharge by means of thermionic emission. Methods of impressing a high voltage on the lamp include series triggering, in which an igniter is used overlapping the high voltage to the electrodes for the main discharge, and external triggering, in which there is an auxiliary electrode that does not contact the discharge space of the main discharge electrodes and the high voltage is impressed on the auxiliary electrode. External triggering has a number of advantages not available in series triggering. In particular, if the high voltage generation section that includes the high voltage transformer is separated from the feeder circuit and located near the discharge lamp, such useful benefits as miniaturization of the discharge lamp ignition device, lower noise, improved safety, and reduced cost can be maximized.
During steady operation, on the other hand, the methods of driving discharge lamps are the direct current drive method and the alternating current drive method. The direct current drive method has a great advantage in that the luminous flux from the lamp is of the direct current type and does not vary with time, and so it is basically possible to apply it in just the same way to both types of projectors described above. The alternating current drive method, on the other hand, has the advantage of using the freedom not found with the direct current drive method of polarity reversal frequency, and so it is possible to control the wear and service life of the discharge lamp electrodes, but there is also a disadvantage, as described below, that arises from the very existence of polarity reversal.
Normally, every reversal of polarity in an alternating current drive causes a slight variation in lamp current, such as a flicker in luminous flux from the lamp or overshoot or vibration. Consequently, if it is applied to the projectors described above that use the time division method, there is the problem that the timing with which the images are produced in succession by time division will not match the timing of the polarity reversals of the lamp's alternating current drive and fluctuation of the display image will appear at the beat frequency; depending on the frequency of the beats this can be very unsightly. It has been necessary, therefore, to devise some way to synchronize the timing of the inverter's reversal of polarity with the rotation of the color wheel, which has the drawback of complicating the discharge lamp ignition device.
In projectors using the DLP method, moreover, the brightness of each color of each pixel of the display image is controlled by the duty cycle of the individual pixel of the space modulation element. With the alternating current drive method, therefore, even if the timing is synchronized, if there is a long period of overshoot, vibration, or other fluctuation of the luminous flux when the polarity is reversed, it becomes necessary to devise either a way to not use the light during that period or a way to control the operation of each pixel of the space modulation element to suppress the fluctuation. The former course has the drawback of lowering the effective efficiency of the light beam, and the latter course has the drawback of greatly complicating the control of the space modulation element in the projector equipment.
The drawbacks related to alternating current drive of discharge lamps can be avoided by minimizing the fluctuation in luminous flux at the time of polarity reversal, but this has not been easy. That is because the discharge lamp ignition device is required not only to reduce the fluctuation of luminous flux at the time of reversal of polarity of the voltage impressed on the lamp, but also to assure steady lighting of the discharge lamp at startup.
It is known that it is effective, in order to assure steady lighting of the discharge lamp at startup, to increase the no-load discharge voltage impressed on the lamp when causing dielectric breakdown in the discharge space by means of impressing a high voltage using either series triggering or external triggering. To achieve this in the case of alternating current drive, it has been common to use what is called “resonant assist,” in which dielectric breakdown in the discharge space is brought about by operating an igniter while causing a series resonance phenomenon at startup to raise the voltage impressed on the lamp.
FIG. 13 is a Figure to explain the principle of resonant assist using conventional series resonance. The discharge lamp ignition device of this Figure has a feeder circuit Ux′ that feeds power to the discharge lamp Ld, a full bridge inverter Ui′ made up of switching elements Q1′, Q2′, Q3′, Q4′ to invert the polarity of the output voltage of the feeder circuit Ux′, and a resonant coil, Lr, a resonant capacitor Cr, and a starter circuit Ut″. At startup, the inverter Ui′ is driven to reverse polarity at the resonant frequency determined by the value of the product of the inductance of the resonant coil Lr and the capacitance of the resonant capacitor Cr or a frequency close to that. The LC series resonance phenomenon thus produced generates a high voltage between the terminals of the resonant capacitor Cr, and that component, together with the starter circuit Ut″ connected in parallel with it, impresses a high voltage on the discharge lamp Ld.
However, with this conventional technology using LC series resonance, it is possible to solve the problem identified above of assuring steady lighting of the discharge lamp at startup, but it is not an adequate solution for the other problem of minimizing fluctuation of the luminous flux at the time of reversal of the polarity of the voltage impressed on the lamp. A brief explanation of the reasons for that is given below.
As described above, the LC resonant frequency is determined by the value of the product of the inductance of the resonant coil Lr and the capacitance of the resonant capacitor Cr, and so, if the inductance of the resonant coil Lr is kept low, the capacitance of the resonant capacitor Cr will have to be a large value. That is because, if both the inductance of the resonant coil Lr and the capacitance of the resonant capacitor Cr are small values, the resonant frequency will be quite high and it will be difficult to operate the inverter Ui′. When the capacitance of the resonant capacitor Cr has a large value, however, if one desires to obtain a sufficiently high voltage by means of resonance phenomena, one will be confronted with the problem of a very high value for the resonant current, which is the current that flows through the series connection circuit of the resonant coil Lr and the resonant capacitor Cr.
If, for example, the switching element Q1′ and the switching element Q3′ are in the ON state, then the resonant current will flow through the entire circuit, including the feeder circuit Ux′ and the inverter Ui′, as shown by the route L01 shown by the broken line in FIG. 13. For that reason, it will be necessary to use high current ratings for the circuit elements in every section in order to withstand the large resonant circuit, and increased equipment size and costs will be inevitable.
Even though the resonant frequency will be very high, one possible measure would be to reduce the value of the capacitance of the resonant capacitor Cr in order to hold down the operating frequency of the inverter Ui′, if operating at a high order of resonance. Even in that case, however, the resonant current would flow along the route L01 shown by the broken line in FIG. 13, as described above, and the resistance of the switching element in the ON state at the time would be relatively large, and so the Q value of the resonant circuit would be small. Therefore, there would be severe attenuation of the resonance and use of high-order resonance would be impossible.
Accordingly, as long as LC series resonance is used, it will be impossible to reduce the inductance of the resonant coil Lr; a large value will inevitably be required. However, at the stage when the lamp startup is completed, regular operation begins, and the lamp's light is in use, a large inductance value for the resonant coil Lr will be a considerable impediment. Generally speaking, in cases where the resonant coil Lr or a large inductance such as an igniter is inserted at a stage subsequent to the inverter, inconvenient phenomena, such as luminous flux overshoot or vibration at the time of reversal of polarity will be encouraged, with the result that it has become necessary to solve the problem of reducing fluctuation of luminous flux at the time of reversal of polarity of the voltage impressed on the lamp.
In the case of external triggering, on the other hand, there is no need for an igniter with high inductance as in the case of series triggering, and so, if the circuit is designed with care not to insert anything like a resonance coil Lr, it will be well suited to avoiding inconvenient phenomena, such as luminous flux overshoot or vibration at the time of reversal of polarity. In the case of external triggering, as existing technology for realization of increased no-load discharge voltage to impress on the lamp when dielectric breakdown in the discharge space is brought about as described above, Japanese pre-grant patent publication 2003-092198 (U.S. Pat. No. 6,661,184 B2) describes a method of impressing high voltage on the pair of electrodes for main discharge by at least partially overlapping the period when an external triggering starter is generating high voltage, which can realize the anticipated function.
In that technology, however, the no-load discharge voltage increases along with the production of high voltage by the starter, and so, after dielectric breakdown succeeds and the starter ceases operation, there is also an end to the increase of no-load discharge voltage impressed on the lamp when the dielectric breakdown was brought about in the discharge space. In order to maintain a glow discharge, therefore, it is necessary that the feeder circuit directly generate a no-load discharge voltage of higher voltage than the glow discharge voltage. That being the case, since the inverter is located at a stage subsequent to the feeder circuit, it is necessary to build the circuitry with elements that can withstand the high-voltage no-load discharge voltage.
However, in addition to the cost of the FET or other switching elements that make up the inverter increasing with their voltage resistance, loss is greater and heat-radiation countermeasures are more costly. These factors increase the cost of the discharge lamp ignition device as a whole and make miniaturization impossible.
Other related devices are described in Japanese Pre-Grant Patent Publication Nos. H03-030291, 2003-217888 and 2004-327117.